1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory device including a variable resistive element having a first electrode, a second electrode, and a variable resistor made up of a metal oxide and sandwiched between the first electrode and the second electrode, in which the metal oxide is an insulator in an initial state, its resistance is decreased by a forming process, its resistance state changes between two or more different resistance states by a voltage applied between the first electrode and the second electrode after the forming process, and the resistance state after being changed is maintained in a nonvolatile manner.
2. Description of the Related Art
In tandem with the penetration of a mobile device such as a portable electronic device, a flash memory has been widely used as a high-capacity and inexpensive nonvolatile memory which can hold stored data even in a power-off state. However, recently, since the flash memory sees limitations in miniaturization, a nonvolatile memory such as an MRAM (Magnetoresistive Random Access Memory), a PCRAM (Phase-change Random Access Memory), a CBRAM (Conductive-bridging Random Access Memory), or a RRAM (Resistance Random Access Memory) has been actively developed. Among the nonvolatile memories, the RRAM attracts attention because high-speed writing can be performed by an applied voltage, a simple binary transmission metal oxide can be used for its material so that production is easy, and affinity with an existing CMOS process is high.
It has been conventionally reported that resistance change is caused by an applied pulse voltage in the many metal oxides serving as resistance change materials which can be used in the RRAM. For example, a resistance switching element (variable resistive element) can be formed by sandwiching both ends of a metal oxide thin film formed of a ternary perovskite material such as PrxCa1-xMnO3 (PCMO), or Ni, Co, Ti, Fe, Cu, Ta, Hf, Zr, Nb, or Al between metal electrodes (refer to W. W. Zhuang, et al., “Novell Colossal Magnetoresistive Thin Film Nonvolatile Resistance Random Access Memory (RRAM)”, IEDM Technical Digest, pp. 193-196, December, 2002, and Baek, I. G. et al., “Highly Scalable Non-volatile Resistive Memory using Simple Binary Oxide Driven by Asymmetric Unipolar Voltage Pulses”, IEDM Technical Digest, pp. 587-590, 2004). Hereinafter, for the convenience of the description, the resistance switching element used in the RRAM is referred to as the “variable resistive element” to be discriminated from a resistance change element used in a memory other than the RRAM. Among the above materials, as for what combination of metal oxide material and metal electrode material results in optimal characteristics, some empirical knowledge has been accumulated. For example, it is known that preferable switching can be implemented by using a material having a great work function such as Pt as the electrode, for the n-type metal oxide such as an oxide of Ti and Ta, and a material having a small work function such as Ti or Ta as the electrode, for the p-type metal oxide such as an oxide of Co or Ni. Therefore, it is considered that the resistance switching action of the RRAM is preferably performed in a junction interface having a Shottky barrier between the metal oxide and the electrode (refer to Japanese Patent No. 4088324). Meanwhile, it is known that it is important to appropriately control a value of a load resistance connected to the variable resistive element in series, with respect to each action mode of the element, and to appropriately distribute the applied voltage between the variable resistive element and the load resistance.
In addition, as for a mechanism of a resistance change of the metal oxide which changes the resistance by the applied voltage, it is considered that the resistance change is caused by generation and extinction of an oxygen defect or movement thereof by an electric field in the oxide in the cases of the perovskite material, Ti oxide, or Ni oxide.
Meanwhile, various tests (such as a function test or a quality test) are performed on the nonvolatile semiconductor memory device such as the RRAM before shipment, and only a chip or a block (a part of the memory cell array) which has passed the test is shipped as a product. At this time, information specific to the individual product (such as relieving information in a case where a memory cell in a part of the memory cell array is used for the redundancy relieving, information such as a defect block address in a case where a part of a defect block is not used, or product information (such as production number) and the like) is programmed in a nonvolatile memory element provided for storing the specific information, to perform redundancy relieving for the defect memory cell, inactivate the defect block, and keep the product information. Meanwhile, as for the memory cell array for storing the user data, all of the memory cells are uniformly set in the same storage state and specific information is not stored at the time of normal shipment.
The nonvolatile semiconductor memory device to be shipped as the product is generally housed in a package by resin sealing and the like, and on the user side, soldered on a predetermined substrate and incorporated in a final product. At this time, in a case where the product specific information stored in the nonvolatile memory element for storing the specific information is unexpectedly written due to a high-temperature treatment such as a solder reflow treatment (such as high-temperature treatment performed at about 260° C.) on the user side, there is a possibility that the redundancy relieving and the inactivation of the defect block cannot function normally, which causes an action defect. In addition, when the product information is written, correct history information at a production stage cannot be known when defect on the market occurs, so that it is difficult to analyze the defect. Furthermore, when the data of the memory cell array for storing the user data is partially written, the user could have a suspicion that the product is a used product in which some data is programmed, or a defective product.
Thus, the present inventor has examined whether or not there is a possibility that the resistance state (stored information) of the variable resistive element is written due to the high temperature treatment after shipment on the user side, using a variable resistive element including a first electrode, a second electrode, and a variable resistor made up of a metal oxide and sandwiched between the first electrode and the second electrode, in which the metal oxide is an insulator in an initial state, its resistance is decreased by a forming process, its resistance state changes between two or more different resistance states by a voltage applied between the first electrode and the second electrode after the forming process, and the resistance state after being changed is maintained in a nonvolatile manner. Thus, it has been confirmed that when a resistance decreasing action to change the resistance state of the variable resistive element from the high resistance state to the low resistance state is performed under a normal program condition (standard program condition set to satisfy product specification), the resistance of the variable resistive element is increased by more than 2 times within a several hours after the variable resistive element programmed into the low resistance state has been left an unbiased state under a high temperature of 150° C. or more, and it has been confirmed that the higher the temperature is, the faster the resistance increase is.
Furthermore, an earnest study by the inventor of the present application has found that the resistance change from the low resistance state to the high resistance state after being left at the above high temperature can be considerably prevented to an allowable degree in practical use by further decreasing the resistance of the variable resistive element under a program condition to make the resistance state lower, as compared with the normal program condition. Furthermore, as for the variable resistive element which has been programed into the high resistance state under the normal program condition, even when it is left at the high temperature similarly to the above, the large resistance change which is problematic in practical use is not confirmed although the resistance is a little increased (such as several %).